Significance Bartonellosis is emerging as an important opportunistic infection in patients withAIDS. Bartonella henselae is the etiologic agent of cat-scratch disease, while B. quintana is the agent of "trench fever", both self-limiting infections in immunocompetent hosts. Bartonella infection in AIDS patients has been associated with a severe life threatening proliferation of vascular tissue affecting multiple organ systems; a condition referred to as Bacillary Angiomatosis (BA). Objectives Utilize the SIV/macaque model system to determine the pathogenesis of Bartonella infection in SIV-infected, immunocompromised macaques, and evaluate the SIV/macaque system as a potential model for BA. SIV-infected animals, at various stages of immunodeficiency, will be inoculated intradermally with B. henselae or B. quintana organisms, and monitored for onset, duration, immune response, and clinical outcome of bacteremia. Results Two SIV-infected macaques (CD4+ lymphocyte count = 400-500/ul and 150-250/ul respectively) inoculated with B. quintana, developed prolonged bacteremia (>120 days) associated with strong antibody response. No clinical signs or systemic affects associated with the bacteremia were observed, other than a transient fever and skin lesions at the site of inoculation 1-2 weeks post-bartonella infection. One SIV-negative rhesus inoculated with B. quintana seroconverted, but exhibited only a transient bacteremia, and no clinically apparent disease. Two rhesus, one SIV-positive (CD4+= 150-250/ul), and one SIV-negative, inoculated with B. henselae, did not develop detectable bacteremia or immune response, and neither animal exhibited any clinical signs of illness. Two SIV+ animals died of SIV/AIDS unrelated to bartonellosis, and no evidence of BA was detected at necropsy. BA may be a very late manifestation of chronic bartonellosis in immunocompromised hosts, and long-term follow-up of these animals is required. Future Directions Additional inoculations with B. quintana and B. henselae will be performed. Attempts to use SIV-infected macaques with low, but stable CD4+ cell counts, to increase the likelihood of long term survival. KEYWORDS bartonella, bacillary angiomatosis; SIV; AIDS; macaque